Field help (BPEVACBPC) 

The customer tax rule by default is stored in the Customer record.
In the case of a multi-companies management, the same customer can be subject to a different tax rule according to the company they deal with.
Use this field to enter exceptions and to set the customer tax rule by company.
Only the tax rules in phase with the legislation and the group of the indicated company can be entered.
SEEREFERTTOThe general principles linked to the multi-legislation setup are detailed here.